One of problems is I could not determine where the canna is growing. This is often important because certain pathogens are more prevalent in specific zones. The necrotic symptoms appear to me to be more closely aligned with a bacteria leaf disease (specifically-bacteria leaf spot).
These symptoms could be the result of a virus infection but does not appear to be either CaYMV or CaYSV.
Canna rust (fungal) is most prevalent in the Southern areas of the U.S. but can occur further north but most often in late summer and the fall season. Rust always begins on the lower side of the leaf surface with the onset of red postulates. Rust is typically easy to diagnose by observing the underside of the leaf prior to necrosis.
If possible, it would be better if your plant received more hours of sun and prevent overhead watering late in the day. Wet foliage for extended time will augment the onset of both bacterial and fungal disease.
I cannot personally rule out any of the three possibilities.

Plethora87,
I don't believe any of us can give you a definitive answer but even though you have had little rain or do not water the foliage the photo indicates that the lower leaves seem to be the most affected? Do you have dew in the mornings? If so, the lower leaves will be the last to dry. Your limited availability of sun would contribute to the possibility of the lower leaves remaining damp longer.
Do a search for "bacterial leaf spot" and compare the damage to your plants. Bacterial leaf spot is more commonly seen in field grown cannas where the soil splashes or is blown onto the plants lower leaf surface but it can be transmitted in other ways. It is not typically seen in container grown stock unless the foliage remains wet for extended hours. I have seen it in greenhouse seedlings and small cannas where the water collects in the whorl. I have observed this condition often in the field (growing in the dirt).
We still can not rule out rust, virus or even a combination of these problems.
Best of luck,